1. Field of the Invention
The present invention relates to a broad-band optical semiconductor device containing an active layer. In particular, the present invention relates to a broad-band optical semiconductor device which has an extended gain band.
2. Description of the Related Art
Generally, a wavelength division multiplexing passive optical network (WDM-PON) requires a light source that emits light over a broad range of wavelengths. To meet this requirement, a WDM-PON is provided with broad-band optical semiconductor devices, such as semiconductor optical amplifiers (SOA), semiconductor lasers, or other devices that can operate over an extended gain band.
FIG. 1 is a sectional view showing a conventional broad-band optical semiconductor device. The broad-band optical semiconductor device 100 shown in FIG. 1 includes a lower electrode 110, a lower clad layer 120, an active layer 130, an upper clad layer 140, and an upper electrode 150.
The lower clad layer 120 is an n-type compound semiconductor. The lower electrode 110 placed at the bottom of the lower clad layer 120 is connected to the ground and formed from a conductive metal. The active layer 130 placed on the top of the lower clad layer 120 is comprised of the 1st through the nth quantum wells 130-1˜130-N, having a thickness increasing gradually from the 1st quantum well to the nth quantum well. The upper clad layer 140 placed on the top of the active layer 130 is a p-type compound semiconductor. The upper electrode 150, where an electric current is applied, is placed on the top of the upper clad layer 140 and formed from a conductive metal.
FIG. 2 is a graph showing the gain curve as a function of wavelengths of the conventional broad-band optical semiconductor device 100 described above. As shown in FIG. 2, the gain v. wavelength curves 210-1˜210-N of the 1st through the nth quantum wells 130-1˜130-N overlap with one another resulting in a broad-band gain curve 220.
As noted above, the conventional broad-band optical semiconductor device 100 is provided with an active layer 130 that includes multiple quantum wells having thickness that increases gradually to provide an extended gain band. However, one of the problems faced by the broad-band optical semiconductor device 100 described above is that it is impossible to obtain uniform gain. In other words, the broad-band optical semiconductor device 100 shows decrease in efficiency of the gain because carriers (electrons and holes) are injected non-uniformly, depending on the position of a quantum well.
Another problem attributable to the conventional broad-band optical semiconductor device 100 is that use of the active layers containing the quantum wells results in a great change in the gain with respect to the direction of polarization of the incident light.
FIG. 3 is a sectional view of another conventional broad-band optical semiconductor device. The broad-band optical semiconductor device 300 includes a lower electrode 310, a lower clad layer 320, an active layer 330, an upper clad layer 340, a first upper electrode 350, and a second upper electrode 355.
The lower clad layer 320 is an n-type compound semiconductor. The lower electrode 310 comprising a conductive metal is placed at the bottom of the lower clad layer 320 and connected to the ground. The active layer 330 is placed on the top of the lower clad layer 320 and generates optical gain depending on the injection of carriers. The upper clad layer 340, a p-type compound semiconductor, is placed on the top of the active layer 330. The first and the second upper electrodes 350 and 355, which are conductive metals and to which electric current is applied, are placed on the top of the upper clad layer 340. The active layer 330 is divided into a first region 370, to which carriers 360 are injected by way of the first upper electrode 350, and a second region 375, to which carriers 365 are injected by way of the second upper electrode 355.
The first level of current I1 and second level of current I2, different from each other, are applied to the first upper electrode 350 and the second upper electrode 355, respectively. Therefore, the first region 370 and the second region 375 have a different number of carriers, and the first region 370 and the second region 375 have different gain bands.
FIG. 4 is a graph showing the gain curve depending on wavelengths in the above-described broad-band optical semiconductor device 300. As shown in FIG. 4, the gain curve 410 of the first region 370 and the gain curve 420 of the second region 375 overlap with each other, resulting in a broad-band gain curve 430.
As described above, the broad-band optical semiconductor device 300 provides an extended gain band compared to other conventional optical semiconductor devices by varying the carrier number distribution along the light propagation axis. However, the broad-band optical semiconductor 300 is incapable of ensuring sufficient gain band, as change in the carrier number results in a change in the gain band.